Volvo MD7A alternator output

[paulsmith]

the output from my alternator (checked across the battery terminals when motoring at cruising speed) is 14.4 volts and it charges the batteries up to 12.5 volts after a good run of about an hour. There are no diodes or relays in the sytem just an on, off, no 1, no 2,switch and I only have it switched to one battery at a time. is this about right? I thought the batteries (both new) should charge up to almost 13 volts.any thoughts?
Paul

 

[Stemar]

I'm working from memory so I may not have the figures quite right, but I don't think you'll find them too far off.

12.5v no-load voltage suggests your batteries are only about half charged. If they don't get fully charged, they'll knacker pretty quickly.

A lead-acid battery in good nick should give 13.2v when fully charged, and not under load. Your altenator will build up to 14.4v relatively quickly, then maintain this voltage for some time as the bulk of the charge goes into the battery(ies).

An hour's charge on a 50amp alternator will replace about 35-40 amp/hours use. In a car, you can reckon on taking around 15mins of daytime driving to put back what you use to start the engine, so it's probably going to be much the same on a boat. A night's sailing with nav lights, gps, etc., could easily use more than that, as could a day's sailing with vhf, fm radio/cd then cabin lights during the evening. It's surprising how it adds up, especially if you have blown air heating or a fridge!

I suggest you take your batteries home from time to time and give them a good charge from a mains charger.

 

[boatmike]

The normal charging voltage from an alternator will be around 14 volts. Higher rates are OK but above 14.4 volts gassing will occur requiring more frequent electrolyte level maintenance. The voltage or a normal 12 volt battery when at rest under no load is 12.7 volts although it may take a while to settle to this level directly following charge.

 

[john_morris_uk]

Sadly the standard alternator on the MD7A is not a very efficient unit. Most alternators won't reach their designed output except in the most favourable of conditions and the unit on the MD7A is not the best alternator ever built.

If you really want to know what is happening whith your batteries, try monitoring current as well as voltage. I doubt whether your alternator is pushing more than 10 to 15 amps into the battery after the first few minutes. So that's 10 or 15 amp-hours for every hour of engine run...

Although possible in theory, I have never found measuring the standing voltage of a battery a very good way of accurately assessing its absolute state of charge. It relies on the battery being in good condition and being allowed to settle for a period before the voltage readings are taken. A hydrometer to measure the specific gravity of the cells is fast and accurate, otherwise one of these battery monitoring systems...